The studies were funded by the National Institute on Drug Abuse (NIDA) Dr. Kenneth Kendler and Dr. Carol Prescott at the Medical College of Virginia (Richmond) have examined the patterns of marijuana and cocaine use by female twins and found that genetic factors play a major role in the progression from drug use to abuse and dependence. This finding is based on interviews with 1,934 twins who ranged in age from 22 to 62. The study also found that concordance rates -- both twins using, abusing, or being dependent on drugs -- were higher for identical than fraternal twins. The researchers conclude that for both cocaine and marijuana, genetic factors are responsible for approximately 60 to 80 percent of the differences in abuse and dependence between fraternal and identical twins. Dr. Ming Tsuang, a researcher at Harvard University, has found that in males genetic influences are stronger for abuse of some drugs than for others. Dr. Tsuang and his colleagues studied drug use in 1,874 identical male twin pairs and 1,498 fraternal male twin pairs recruited from the Vietnam Era Twin Registry. The researchers found evidence to suggest that genetic influences contribute to a common vulnerability for abusing marijuana, sedatives, stimulants, heroin or opiates, and psychedelics. Studies that involved male and female twins suggest that genetic factors for drug abuse are stronger in males than in females. NIDA official advises that although these twin studies cannot provide information about the drug abuse risk for a particular individual, they are of significant value in helping define the variations in drug abuse vulnerability in the population. 5 references